Acoustic intensity methods and their applications to vector sensor use and design

by Naluai, Nathan Kahikina.

Abstract (Summary)

Applications of acoustic intensity processing methods to vector sensor output signals
are investigated for three specific cases: acoustic intensity scattering, spatial
correlations of intensities, and conceptual design of a high frequency inertial vector
sensor with a novel suspension. An overview of intensity processing is presented
and the concept of a complex intensity is illustrated. Measurement techniques for
determining the complex intensity spectra from the signals received by a standard
acoustic vector sensor are demonstrated.
Acoustic intensity processing of signals from SSQ-53D sonobuoys is used to
enhance the detection of submerged bodies in bi-static sonar applications. Deep
water experiments conducted at Lake Pend Oreille in northern Idaho are described.
A submerged body is located between a source and a number of SSQ-53D sonobuoy
receivers. Scalar pressure measurements change by less than 0.5 dB when the
scattering body is inserted in the field. The phase of the orthogonal intensity
component shows repeatable and strong variations of nearly 55o.
The classical solution for the spatial correlation of the pressure field is presented.
The derivation techniques are expanded to derive previously unsolved
analytic forms for the spatial correlations of separated intensity field components
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based on combinations of the solutions for various pressure and velocity components.
Experimental validation of these correlation solutions are performed computationally
and in an underwater environment. The computational experiments
are designed to test highly controlled variations to the idealized case (e.g. sound
field content, transducer phasing issues, additive output noise, etc.). Additional
verification is provided from physical tests measuring the correlations between a
pair of acoustic vector sensors in a large reverberant tank which is excited acoustically
with broadband noise. The results successfully corroborate the derivation
methods for correlations of individual vector sensor components and for intensity
processed vector sensors.
A conceptual design for an improved neutrally buoyant underwater acoustic
vector sensor is proposed. The design incorporates a novel suspension design that
can be rigidly mounted to an existing support structure without affecting the
sensor’s performance. The sensor response is shown to be locally frequency independent
across a frequency range from 1.0 to 30.0 kHz, with a total phase variation
of less than 0.3 degrees. Fundamental limits of signal detection are determined to
be inherent primarily in the velocity sensing component. These limits are shown
to be roughly equivalent to sea-state zero at 1 kHz and increasing to sea-state 1
or higher for frequencies greater than 10 kHz.
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